1. Field of the Invention
This invention relates to an electromagnetic induction type inspection device and method for detecting defects such as void and foreign matter, abnormality, electric or magnetic characteristic changes of quenching and annealing, and minimal changes in electromagnetic characteristics of a material being tested by a change of electromagnetic induction. More particularly, this invention relates to an electromagnetic induction type inspection device and method for detecting defects in test materials such as structural parts in automobiles, industrial machines, rails, airplanes and architectural component elements for electric power plants, plant piping in oil factories, bridges, and buildings by placing the test material in a magnetic field induced by an exciting coil and measuring a change of electromagnetic induction.
2. Description of the Related Art
A change of magnetic flux is caused by placing a material object in an alternating magnetic field, resulting in a change of inductance of a coil placed in the same magnetic field. The inductance changes with factors such as electric conductivity, magnetic permeability, size and position of the material object in the magnetic field. As the material object is placed in the magnetic field provided some of known factors are kept constant, the other unknown factors can be determined. There have so far been a variety of electromagnetic induction type defect inspection devices having a nondestructive inspection function of identifying an object being inspected and detecting the presence or absence of the inspection object.
As one typical conventional inspection device of this kind, there has been proposed an inspection device 100A using self-inductance as illustrated in FIG. 1 (U.S. Pat. No. 5,432,444). The inspection device 100A comprises a bridge circuit 102 having an electromagnetic coil 101 for producing a magnetic field by application of alternating current. The electromagnetic coil 101 excited by the alternating current from an AC source 103 is equal in inductance to an inductor L of a gage arm of the bridge, while the resistors R1 and R2 having equivalent resistance in the other bridge arms are balanced, so that the output Vout of a differential amplifier 104 connected to the output points Pa and Pb is zero in theory in an equilibrium state of the bridge.
However, when placing an inspection object S in a magnetic field (magnetic flux fL) generated by the electromagnetic coil 101, the self-induction inductance of the electromagnetic coil 101 changes. As a result, the inductor L and the electromagnetic coil 101 in the bridge circuit 102 are unbalanced in inductance, thereby to create a potential difference across the output points Pa and Pb and then produce an output Vout corresponding to the coefficient of induction of the inspection object S. By using data on change of the output Vout, it is possible to easily recognize the material and size of the inspection object and even the velocity of the object moving in the magnetic field. Also, a foreign substance such as contaminant in the known inspection object can easily be found. For that purpose, the bridge circuit of the inspection device should have balance in inductance in the state that the inspection object is brought close to the electromagnetic coil 101 while placing a standard specimen opposite to the inductor L.
There has been hitherto known another typical inspection device 100B making use of mutual inductance as shown in FIG. 2. The inspection device 100B comprises an exciting coil (primary coil) 106 excited by being applied with alternating current from an AC source, paired detection coils (secondary coils) 107a and 107b inducing electromotive forces with application of the magnetic flux of the exciting coil 106, and a differential amplifier 108. The detection coils 107a and 107b are wound in opposite directions and connected differentially in series so as to allow the electromotive forces induced in the detection coils to cancel each other out due to the uniform magnetic flux fL of the exciting coil 106 in the static state. That is, the differential voltage across the output points Pc and Pd of the detection coils 107a and 107b (output Vout of the differential amplifier 108) becomes theoretically zero in the static state.
The inspection device 100B generally has an inspection space 109 between the exciting coil 106 and the inspection coils 107a and 107b, through which the inspection object S passes for the purpose of inspection. As the inspection object S gets across the magnetic flux fL of the exciting coil 106 to cause change of flux linkage on the detection coils 107a and 107b, the bridge comes to nonequilibrium, consequently to create the differential output Vout. Thus, it is possible to determine the material and size of the inspection object S or find defects such as void and foreign matter.
The other inspection device 100C (magnaflux inspection device) hitherto known has been proposed in Japanese Patent Application Publication HEI 10-288605(A), as shown in FIG. 3. The inspection device 100C has a detection sensor 114 and a signal processor 133, so that an AC signal created by an oscillating circuit 122 of an AC signal generator 123 is applied to the exciting coil 111 of the detection sensor 114 through a constant current circuit 121. A phase-difference output 130 and differential output 131 to be outputted from a phase sensitive detector 128 and a differential amplifier 129 are obtained from changes of signals issued from the detection coils 113 through amplifiers 124 and 125 and phase adjusters 126 with reference to a signal from the oscillating circuit.
As seen from the above, the conventional electromagnetic induction type inspection device 100A in which the non-equilibrium state of the induced inductance in the balancing circuit having the induction coil is determined in the form of the differential voltage, is required to detect the change of the electromotive force induced by magnetic flux interlink of the inspection object with a high sensitivity.
However, the self-inductance type inspection device 100A has a little bit of rate of change of self inductance (difference between the specified inductance and the inductance in changing), and thus, cannot detect the inductance unless the inspection object has sufficiently large dielectric constant or is made of material capable of bringing about a large change of magnetic field such as ferromagnetic material. That is, the conventional inspection device has a low sensitivity, so that it cannot be applied to discrimination of the material of the inspection object and detection of the inspection object such as of nonferrous metal having a small rate of inductance change.
Meanwhile, the electromagnetic induction type inspection device 100B using mutual inductance, which has the inspection space 109 between the exciting coil (primary coil) 106 and the inspection coils (secondary coils) 107a and 107b. The induction efficiency of the detection coils 107a and 107b is in inverse proportion to the dimensions of the inspection space 109 (to be more precise, distance d from the exciting coil to the detection coils). Thus, even though this conventional inspection device is desired to have the large inspection space (inspection space) between the exciting coil and the detection coils to inspect the large object, the inspection space is limited to be made large, so that the large object cannot be inspected substantively. One of the main reasons for being able to make the inspection space large is that the detection resolution of the inspection device is diminished with the dimensions of the inspection space 109 even while enhancing the exciting performance of the exciting coil 106, consequently to make it difficult to detect a minute change.
The inspection device using mutual inductance has another inevitable disadvantage. For instance, the inductance of the detection coil 107a, which should be a standard inductance in the circuit, varies more or less with the change of the magnetic flux, resulting in cancellation of the electromotive force induced by the detection coil 107a due to the simultaneously-varied electromotive force induced by the detection coil 107b. The influence of the cancellation of the electromotive force is intricately varied with the relative position of the inspection object to the coils, consequently bringing about unignorable detection error in magnitude of the change of the expected electromotive force.
The inspection device (magnaflux inspection device) 100C illustrated in FIG. 3 is featured by two sensors and phase adjusting circuits in circuitries for generating a phase difference output 130 and a differential output signal 131. As an amplitude factor should be excluded in carrying out the phase adjusting, the changes of the amplitude and phase outputted from the sensors cannot faithfully be reflected in the inspection device 100C. Besides, a signal appearing in the output from the exciting coil irrespective of the inspection object to be inspected is inevitably amplified with the desired signals, so a high sensitive detection cannot be expected.
As noted above, the conventional electromagnetic induction type inspection devices have disadvantages of being inferior in detection accuracy of minute change in the inspection object and not applicable to a subtle inspection such as identification and discrimination of the material of the inspection object. Meanwhile, the aforementioned differential output type magnaflux inspection device has also disadvantages such that the output from the detection sensor interferes mutually with the phase varying in inspecting the object, thus to hinder a high accuracy inspection.